1. Field of the Invention
The present invention relates to a semiconductor power converting apparatus with employment of a semiconductor element and the like. More specifically, the present invention relates to a semiconductor power converting apparatus capable of suppressing a peak voltage during a switching operation without requiring a snubber circuit.
2. Description of the Related Art
IGBTs (Insulated-Gate Bipolar Transistor) known as a typical insulated-gate transistor own low gate power consumption and can be switched in high speeds with a small switching loss. Accordingly, these IGBTs are employed in semiconductor power converting apparatuses having relatively medium and small capacities. Furthermore, these IGBTs are desirably applied to semiconductor power converting apparatuses having large capacities. In general, a jump-up voltage ".DELTA.V" of an IGBT having a snubber circuit is expressed by the following formula (1--1) based upon a capacitance "C" of a snubber capacitor: EQU .DELTA.V=1L/C (1--1)
In other words, this jump-up voltage .DELTA.V is direct proportional to both a switching current I and a root-mean-squared value of a wiring inductance L. Therefore, in the case that a wiring inductance L can be minimized and a switching current I is low in a semiconductor power converting apparatus having a medium/small capacity, since a switching loss E.sub.off of an IGBT is present, this power converting apparatus can be made in a snubberless form, namely a snubber capacitor C is omitted. However, a semiconductor power converting apparatus having a large capacity requires such a snubber circuit for suppressing a peak voltage produced when a large switching current is turned OFF, and furthermore, another snubber circuit for equally sharing a high DC voltage to series-connected semiconductor elements. Thus, switching losses of these snubber circuits would reduce the converter efficiency, namely could not be neglected. Moreover, since the snubber circuits are connected to such a semiconductor power converting apparatus, the cost thereof is increased and this semiconductor power converting apparatus becomes bulky. Also, when a large number of IGBTs are connected in series to each other, both ON timing and OFF timing of all of these IGBTs must be adjusted in high precision in order to equally sharing the voltages to these IGBTs. This requires time and high cost. As a result, very recently, various circuit systems have been proposed. In these circuit systems, the peak voltages are suppressed when the IGBTs are turned OFF. Alternatively, the stational voltage sharing operation for the series-connected IGBTs is uniformly carried out on that any snubber circuit. This recently proposed circuit system corresponds to, as described in Japanese laiid-open patent application No. 11-178318, the gate driving circuit with the basic circuit arrangement such that the zener diode is connected between the collector of the IGBT and the gate thereof, or the series circuit made of the zener diode and the resistor is connected between the collector and the gate of the IGBT.
In this known gate control circuit, the avalanche current will flow when the collector voltage of the IGBT becomes higher than, or equal to the avalanche voltage of the zener diode, and thus, since the voltage of the gate resistor is increased, the peak value of the collector voltage of this IGBT is suppressed. However, in connection with a high withstanding voltage of an IGBT itself, an avalanche voltage of a zener diode would also require several Kilovolts. Further, in order to rise up the gate voltage of the IGBT by an avalanche current, such avalanche currents having values of several to several tens of Amperes are required. In addition, a resistance value of a gate resistor would also require approximately several tens of Ohms, so that the switching loss of this IGBT would be increased.
Conventionally, when the peak voltage of the IGBT is suppressed, since the withstanding voltage of the IGBT is increased, the avalanche voltage of the zener diode connected to the collector of the IGBT must be high. Furthermore, the avalanche current of the zener diode must be increased, or the gate resistance value must be increased instead of increasing of the switching loss. As a result, there are such problems that the higher withstanding voltage of the IGBT cannot be realized, but also the switching loss is increased.
Also, in the case that a plurality of IGBTs which are simultaneously switched are connected in series to each other by employing the conventional circuit system capable of suppressing the peak voltage produced when the IGBT is turned OFF, if the characteristic fluctuations as to the respective circuit elements are not strictly selected and are not made coincident with each other, then the peak voltages produced when the plural IGBTs are turned OFF are fluctuated, and also the stationary voltage sharing conditions are fluctuated. These circuit elements are the IGBTs, the resistor, the zener diodes, and the transistors, which constitute this conventional circuit system. As a consequence, the switching frequency could not be made high.